In a known type of nuclear power reactor the nuclear fuel is contained in a plurality of elongated fuel elements or rods which are grouped together and contained in open-ended tubular flow channels to form separately removable fuel assemblies or bundles. A sufficient number of fuel assemblies are arranged in a matrix to form the nuclear reactor core capable of self-sustained fission reaction. The core is submersed in a fluid, such as light water, which serves both as a coolant and as a neutron moderator.
Typically the fuel rods are formed of a sealed tube containing the nuclear fuel, for example, in the form of a plurality of cylindrical fuel pellets as shown, for example, in U.S. Pat. No. 3,365,371. The tube, sealed by end plugs, thus serves as a cladding to isolate the nuclear fuel from the moderator-coolant and to prevent release of fission products.
Some of the fission products are gases. To accommodate and contain these fission product gases it is known, as shown in the above mentioned U.S. Pat. No. 3,365,371, to provide a space or plenum in the fuel element which is not occupied by the nuclear fuel. The provision of this space or plenum creates the problem of retaining the column of fuel in its desired position because, for proper nuclear performance, it is important that the fuel be in a specified active fuel zone of the reactor core. If the fuel column is not retained in proper position, it is found that some of the fuel may shift into the above mentioned plenum, especially during handling and shipping when the fuel rod may be in other than a vertical position. It is further found that the shifted fuel may cock or wedge against the cladding tube so that it does not return to its proper position in the active fuel zone when the fuel rod is loaded into the reactor core. Various means are shown in the prior art for retaining the column of fuel in its desired position.
Dished shaped disks or washers to retain the fuel column in position are shown in U.S. Pat. No. 3,230,152, the disks being designed to wedge against the inside of the cladding tube. Such fuel retainers do not provide for axial expansion of the fuel column, they are difficult to insert properly and they can produce undesirably high local circumferential stresses in the cladding tube.
Shown in U.S. Pat. No. 3,627,635 is a resilient cup-shaped member which is placed atop the fuel column and is retained in position by frictional engagement with the cladding.
Another fuel column retainer which depends upon frictional engagement with the cladding tube comprises a helical coil spring with end coils which engage the cladding tube as shown in U.S. Pat. No. 3,310,474.
Another type of fuel column retainer, shown for example in U.S. Pat. No. 3,378,458, comprises a helical coil spring which is somewhat smaller in diameter than the cladding tube and is compressed between the top of the fuel column and the upper end plug of the fuel rod. This type of fuel column retainer has gained favor and is widely used. It provides the advantages of reliable and predictable performance and it avoids circumferential stressing of the cladding tube. A problem attendant the use of such a retaining spring is set forth in said U.S. Pat. No. 3,378,458. Namely, when the end plug is welded into the end of the cladding tube there is a danger that the weld will become contaminated with spring material. This is because the spring material (e.g. Inconel-X, steel or the like) and the end plug and cladding tube material (e.g. zirconium) can form a eutectic alloy, the melting temperature of which is lower than either of the base materials. Formation of this eutectic alloy with the weld or fusion area is unacceptable because of unpredictable corrosion resistance, strength and ductility and other unpredictable material properties of the resulting alloys when subjected to the reactor or fission product environments.
A solution to this problem proposed in said U.S. Pat. No. 3,378,458 includes the bending of an end loop on the retaining spring to an angle of ninety degrees so that this loop contacts only the center (and coolest during welding) portion of the end plug. This loop is also plated or coated with a barrier material (e.g. a material such as chromium) to increase the eutectic temperature. This solution has proven effective when properly implemented and it has been used extensively.
However, in the implementation of the foregoing solution for large scale fuel production, several problems have been encountered including difficulties in achieving high integrity plating, handling damage to the plated spring end and difficulties in detecting imperfections and damage so that quality control to the degree of confidence desired is difficult and expensive to achieve.